Metal stock, such as wire, rods, bars and tubes, is formed by rolling, drawing and otherwise shaping the metal both while it is at elevated temperature and after it has cooled. The metal stock cools and cures in a heterogeneous manner such that a degree of warpage and curvature typically develops in the stock.
In the prior art, the stock has been straightened in machines known as cross-roll straighteners. Typically, such machines include one or more pairs of rolls that are disposed in skewed relation and spaced apart from each other. The space between the member rolls of each pair is called a "pass". Where more than one roll pair is used the roll pairs are adjacently arranged such that the "passes" of the roll pairs collectively define a "pass line" along which the stock travels through the cross-roll straightener. At least one of the rolls of each pair are rotatingly driven such that the stock is rotatingly advanced therebetween along the pass line.
In order to straighten the stock, conventional cross-roll straighteners generally apply a concentrated central load between supports to stress the stock beyond its yield point. Such concentrated central loads produce a peak bending moment that is applied in the shape of a helix on the surface of the stock as it rotatingly advances through the machine. Unfortunately, such a peak bending moment nonuniformly flexes the stock such that it is not evenly straightened.
Cross-roll straighteners of the prior art have many work roll arrangements that include various numbers of work rolls. For example, 2-roll straighteners of the prior art generally form a curved pass between a straight or convex roll and a roll having a concave contour. The straight or convex roll deforms the stock against the concave roll. However, such 2-roll straighteners require complex guides to maintain the stock in the roll pass, and have a limited capacity in straightening various sizes and materials.
An improvement in 2-roll straighteners is described in U.S. Pat. No. 3,047,046 in which a concave male roll is located on the convex side of the curved pass and is smaller in diameter than the female roll. Both rolls are contoured so that the stock is wedged between the rolls at the ends of the pass and thereby maintained in the curved pass without guides. The central portion of the curved pass in the straighteners of U.S. Pat. No. 3,047,046 has a substantially parabolic curvature such that the bending moment on the stock is substantially that of a uniformly loaded beam. However, because both rolls were concave and were contoured to contact the stock throughout the curved pass, a relatively large roll angle change for the female roll was required whenever a different diameter stock was straightened. Specifically the roll angle adjustment for the female roll is much greater than the corresponding adjustment for the male roll. Consequently, excessive slippage between the two rolls and the stock occurred for certain stock sizes and materials where the opposed roll had substantially different contact angles with the stock.
In cross-roll straighteners having more than two work rolls, the rolls are generally arranged in a plurality of cross-roll pairs, each forming a straight pass. The cross-roll pairs are arranged in laterally offset fashion so as to flex the stock as it advances along the pass line. Such arrangements have generally been found to be faster and more efficient than 2-roll straighteners. Typical examples are 5-roll and 6-roll straighteners. Conventional 5-roll straighteners have two pairs of cross-rolls and an intermediate single bending roll. Such 5-roll straighteners generally provide greater leverage for bending the stock than a single curved pass 2-roll straightener. However, 5-roll straighteners also require additional guides to maintain the stock in proper position as it passes through the machine.
Conventional 6-roll straighteners have three pairs of cross-rolls of equal diameter that form straight passes. The pass line is defined by a central pass that is laterally offset from the line between the end passes. As the stock moves along the pass, it is flexed between the central roll pair and supported by the two end roll pairs in a manner similar to a simple beam having a single load between two supports. 6roll straighteners can generally be operated at higher speeds and without guides. 6-roll straighteners have generally been used for straightening tubular stock of small diameter and medium to low strength material.
However, these straighteners have required a longer bending span than 5-roll straighteners and, therefore, cannot uniformly flex all stock sizes to a minimum radius of curvature as required to flex most of each section to or beyond the yield point of the material. Some restraining action on the stock occurs in the endroll passes, as evidenced by the smaller slope of stock deflection at the ends of such straighteners. However, this restraining action is insufficient to provide effective reverse bends for all portions of the stock, or to eliminate problems with the tables, particularly the outlet end table. Such moderate restraining action on the stock, together with the lack of guidance in the roll pairs, does not produce sufficient uniform flexure for good straightening and also limits the stock capacity of the machine and the throughput speed of the stock. Another problem has been that the passes in conventional 6-roll straighteners do not provide positive guidance for the stock.
Because of the longer bending span, non-uniform flexure, and poor guidance of the stock, prior multiple pair cross-roll straighteners were much less effective in straightening the stock than 2-roll straighteners. In particular, the female roll of the central roll pair had a symmetrical roll contour that did not conform to the surface of flexed stock. The concentrated bending loads also resulted in concentrated roll pressure on tubular stock that is sufficient to oval thin wall tubular sections.
Prior attempts have been made to achieve more uniform distribution of the straightening loads in crossroll straighteners, particularly for applications involving larger diameter tubing having a relatively thin wall. The examples of such prior art, as shown in U.S. Pat. Nos. 2,376,401; 2,757,707; and 3,008,510, however, have a straight central roll pass that interferes with the curved, flexed stock.
In the prior art, rolls having various multi-sectional contours have been developed to improve the flexure of the stock and the machine capacity or straightening various stock sizes and materials. For example, U.S. Pat. No. 4,056,958 includes a roll having a two-section curvature defined by hyperboloids. However, since the contour is not symmetrical, there rolls cannot be used to flex and straighten a large variety of stock sizes and materials. U. S. Pat. No. 2,655,194 describes a roll having a contour with five sections wherein the outer sections are used for straight passes of the stock, the central section is defined by a cylinder of smaller diameter than the outer sections, and the intermediate sections are portions of circular cones. The central section is used to form a curved pass for straightening smaller stock sizes. The intermediate sections are selected so that the roll can be manufactured in a single set up with a specially contoured grinding wheel. The problem with such multi-sectional contoured female rolls has been that they do not provide a curved pass for a broad range of stock sizes and, particularly, for larger stock sizes. Furthermore, they generally do not provide a curved pass for smaller stock of high strength material. In the prior art, straightening stock in a curved pass has required different roll angle settings for the female roll and the male roll. The large difference in roll angle settings required to straighten small diameter or high strength stock as well as larger stock sizes in the same pair of rolls has greatly limited the capacity of conventional curved pass straighteners.
Other difficulties also existed with the prior art cross-roll straighteners. For example, in roll angling mechanisms such as shown in U.S. Pat. No. 3,604,236, the yoke tended to slip during the straightening operation so that the roll support would move axially after the angling screws were tightened. Other mechanisms tended to vary due to play in the adjustment screw threads or gears. In conventional 6-roll straighteners, angling of the rolls was somewaht cumbersome in that it required adjustment of twelve handwheels to angle and lock the rolls in position.
As another example, the roll brackets were expensive and difficult to maintain. In removing the rolls, bearing caps had to be removed and retainer screws loosened, thus exposing the bearing. In order to obtain accurate axial roll adjustment, bearing retainers had to be loosened and shims added or removed.
As still another example, the main frames of conventional straighteners such as shown in U.S. Pat. Nos. 3,540,251 and 3,604,236 absorb moments from lateral forces at the top or bottom of loosely fitted tie rods that are deflected by lateral forces and vibrations. The tie rods are weakest where the moment is greatest and the restraining moments in the upper plate and the base are excessive due to the short distance of restraint.
Accordingly, to provide higher operating speeds and improved straightening through uniform flexure, there was a need in the prior art for a straightener having the advantages of a curved pass 2-roll straightener but with greater straightening leverage and better guidance, such as found in 5 and 6-roll straighteners. In addition, there was a need for female roll suitable for use in a curved pass, but having a concave contour that would avoid excessive slippage between the rolls and stock for a broad range of stock sizes and materials.